This center is one of about 45 institutions in North America and Europe participating in the Biogen-sponsored Phase III clinical protocol. The primary objective of this trial is to determine if weekly intramuscular injections of recombinant interferon beta-1a (Avonex) is effective in slowing the progression of secondary progressive multiple sclerosis (MS) when compared to placebo. Progression will be assessed by a composite outcome measure consisting of the timed 25 foot walk, 9-hole peg test and Paced Auditory Serial Addition Test (PASAT 3). Secondary objectives of this study include: 1) determining the safety of Avonex administered at a weekly dose of 60 mcg in patients with secondary progressive MS; 2) determining the effect of active therapy on the number of gadolinium enhanced lesions found on annual magnetic resonance imaging (MRI); 3) evaluating the effect of active therapy on the number of new and enlarging lesions found on T2-weighted annual MRI, and 4) evaluating the extent of confirmed change from baseline Expanded Disability Status Scale (EDSS) score for actively treated patients compared to those on placebo. Approximately 436 patients with secondary progressive MS were recruited to this study.